This invention generally relates to electronic systems and in particular it relates to start up circuits for high speed bias generators.
A very important part in the design of operational amplifiers is the bias generator. Bias generators provide a reference current that sets the quiescent current for the given design. Usually bias generators can be independent of supply voltages, so references like Vbe (base to emitter voltage) or Vt (threshold voltage) are used. One important part of the design of the bias generator is the startup circuitry. Start up circuits will force the bias generator to operate in the non-zero state. They do this by putting a small current that will force the circuit to operate and keep it from turning off.
In the world of high speed circuits an essential requirement for bias generators is to be able to tolerate the high frequency feed through of the signals that will ripple back from the main circuit. The signals that ripple back can cause the bias generator current to spike up or to almost turn off. The bias generator has to be able to absorb these signals and recover in a very short amount of time. As soon as the bias generator starts to turn off, the start up circuit should catch up bringing the bias generator current back to its normal state. The start up circuit has to be fast for a very high-speed circuit. What one would ambition is a bias generator that could speed up as a result of a fast transient but after that overshoot it never undershoots, i.e., a 50-60 degrees of phase margin. This is why in high-speed design extra compensation to the bias generator is not desirable.
FIG. 1 shows a prior art PTAT bias generator with a high speed start up circuit. The start up circuit 20 is always providing a current to the bias generator 22, as opposed to xe2x80x9cnon-high speedxe2x80x9d start up circuits which are disconnected when they are not needed. The reason for having the circuit providing a constant start up current is to fulfill the requirement for a fast start up circuit when dealing with high-speed signals. One thing to keep in mind when designing the startup circuitry is not to limit the power supply""s voltage range beyond what the core circuitry already does. Emitter degeneration resistors 30 and 32 (R3) are usually used to improve the matching of the transistors in the start up. Typically, the voltage drop across them is no more than 10 VT where VT=kT/q. For voltage drops larger than 10 VT the improvement achieved is almost insignificant and it starts to limit the power supply voltage range.
Prior art start up circuits such as the one shown in FIG. 1 have the problem of being power supply dependent. The start up reference current in transistor Q2 will be determined by the difference in voltage between the power supplies Vcc and Vee minus one diode drop across transistor Q1 divided by a set of resistors R1 and R4 (R1+R4, of which R4 usually dominates). This is imposed into the base of transistor Q2 setting the start up current. Notice how the start up current will also be power supply dependent. This can be a problem on a wide supply voltage application. If the start up current becomes large it will introduce a substantial error in the reference current as a result of the impedance drop in the PNP mirrors 24 and 26. This error will affect the currents throughout the whole circuit and increase the power consumption. On the other hand, if it gets too small it will fail to keep the bias generator from recovering fast after a fast transition ripples back to it nearly turning it off.
A start up circuit includes: a diode; a first transistor coupled in series with the diode; a first resistor coupled in series with the transistor; a second transistor having a control node coupled to a control node of the first transistor and coupled to a node between the first transistor and the first resistor; and a second resistor coupled in series with the second transistor such that a current in the second transistor is independent of a voltage applied across the diode, the first transistor, and the first resistor.